It is known that the mechanical strength of assemblies is considerably improved by the effect of a pre-load as high as possible introduced by tensioning the fixing elements, very often formed by threaded elements. In this case, tensioning of the screw is provided by the screwing torque, which may be obtained by means of a spanner or of a tightening tool of dynamometric type. However, this solution requires regular calibration of the equipment and this is why it is now preferred to use fixing systems themselves producing the desired screwing torque. Thus, systems are known in which the nut includes a driving part which is designed so as to break when a predetermined screwing torque is reached. Such systems are relatively expensive.
Moreover, the unscrewability of threaded systems is generally provided by permanent deformation of the nut which produces a resistive torque by friction on the screw. Now, when a high and given pre-load is desired in the assembly, this resistant torque is opposed to the screwing torque and therefore introduces a dispersion factor prejudicial to the effective tensioning of the screw.